FLC OF A DC9-30 WERE DSNDING AS INSTRUCTED BY ATC WHEN THE ARTCC CTLR QUERIED THE FLC REGARDING THEIR ALT AND THAT THEY WERE NOT CLRED FROM ASSIGNED ALT. ANOTHER COMPANY ACFT CONFIRMED THAT RPTR HAD BEEN GIVEN THE CLRNC.

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IN CRUISE WE WERE GIVEN INSTRUCTION TO DSND; PLT'S DISCRETION; TO 12000 FT. SHORTLY THEREAFTER; TO CROSS TAMMY INTXN (39 DME FROM MEM) AT 12000 FT. WE RPTED OUT OF FL230 FOR 12000 FT. LATER; THE CTLR ASKED WHAT ALT WE WERE SUPPOSED TO BE AT. AT THIS TIME WE WERE PASSING FL205 IN DSCNT. WE GAVE THIS AS OUR ALT. HE REPLIED THAT WE SHOULD BE AT FL230. ANOTHER FLT (ACR Y) CAME ON FREQ AND AGREED WITH US THAT HE (ATC) HAD GIVEN US THE ALT CHANGE. THIS WAS THE LAST THAT WE HEARD OF THE ALT MISUNDERSTANDING FROM ZME. AT THIS TIME; INBOUND TO MEM; THERE WERE FLTS ACR X (OURS); ACR Y AND ACR Z. FACTORS: POSSIBLE CTLR CONFUSION DUE TO SEVERAL FLTS WITH SIMILAR OR SIMILAR SOUNDING FLT NUMBERS. AS A NOTE: WE HAD SIMILAR PROB THE NIGHT BEFORE (MEM-JAN). POSSIBLE CORRECTIONS: 1) HAVE ALL AIRLINES CHK FOR SIMILAR FLT NUMBERS. 2) ELIMINATE 4 DIGIT FLT NUMBERS. IN MY OPINION THEY ARE 'HARDER' TO HEAR IN THE BUSY ATC ENVIRONMENT. THEY ALSO SEEM TO GET 'CHOPPED OFF' AT THE START AND STOP (ENDING) OF A XMISSION. 3) EVERYONE (ATC AND PLTS) USE GOOD RADIO PROC -- ESPECIALLY ONE'S COMPANY CALLSIGN. ACR X; ACR Y; ACR Z; ETC. THESE 2 CAUSE PROBS DURING BUSY TIMES.